416 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



or needles of feldspar which are composed of minute stout prisms attached 

 to one another end to end, in parallel position, producing a rude microscopic 

 rod with uneven sides. These prisms are elongated parallel to the vertical 

 axis, and for the reason just given are optically sometimes negative and 

 sometimes positive, or they may all be positive. They form branching 

 arborescent growths, and constitute rays of spherulites, usually of consid- 

 erable size. A phase of this kind of spherulitic growth is shown in PI. 

 LVI, fig. 3. Between the feldspar rays there are minute grains of tridy- 

 mite, which are often clustered in small spherical pellets with cavities 

 between them. Such spherulites are porous. In the more coarsely crys- 

 talline spherulites, with large cavities, the same kind of feldspar crystals 

 may be seen with a low magnifying power. Both kinds of feldspar prisms 

 often occur in the same spherulite. 



Some spherulites are composed of a dense micrographic spherulite 

 at the center, which passes outward into the branching variety, which may 

 be porous to a greater or less degree, the free silica being in the form of 

 tridymite. In some areas of the tridymite grains between comparatively 

 coarse prisms of feldspar (PI. LVI, fig. 2) there are crude spherulitic aggre- 

 gations of tridymite, probably composed of interpenetrating tablets. 



LITHOPHYS^E. 



The microstructure of the lithophysa? can not be studied so easily as 

 that of the more compact spherulites, because of the slight coherence of 

 the crystals composing them and the difficulty of preparing thin sections. 

 But in many cases the component crystals are large enough to be seen with 

 a pocket lens, and their character and arrangement can be made out. It is 

 evident in the case of hollow or gaping porous spherulites, which are one 

 phase of lithophysse, that the mass consists of short prisms of orthoclase or 

 sanidine attached to one another end to end, in a manner already described 

 for some spherulites. These jointed rods of feldspar radiate from what was 

 the center of the spherulite, the gaping appearing to have taken place after 

 their crystallization. With them are associated tridymite, cpiartz, and 

 fayalite. When there is no marked banding in the surrounding rock mass, 

 the hollow spherulite may have the form represented in cross section in 

 PL LVII, fig. 1 . When pronounced banding is present in the groundmass, 



